Luminophore layers that operate as storage film (i.e., that store x-ray information) can be used for the generation of x-ray exposures. Such storage films are particularly used in digital radiography and mammography. The x-ray information is obtained by a process that begins with the body to be examined being traversed by x-ray radiation. After this irradiation, the x-ray radiation impinges on the storage film where it effects changes on storage elements integrated into the storage film. The number of the storage elements thereby set depends on the intensity of the impinging x-ray radiation. Due to the spatial distribution of the storage cells across the storage film, an x-ray exposure with the size of the exposed part of the storage film thereby results.
The storage elements of the storage film must be read out for generation of electrically-processable image data or image data visible to the human eye. The contents of the storage elements can be optically established. For readout, they are radiated with light of a specific wavelength and thereby optically excited. Such an excited storage element emits light of a specific wavelength in the event that it was charged or set beforehand via the absorption or x-ray radiation. The intensity of the emission light thereby depends on the number of set storage elements and therefore forms a measurement for the previously-absorbed x-ray radiation. The emission light is of a relatively lower intensity and is therefore measured with high-sensitivity detectors, for example, with photomultipliers.
The exposed storage film is read out pixel for pixel to generate an x-ray exposure. Electronic image data or image data perceivable by the human eye are generated from the read-out information. Due to the optical readout of the storage film, very high requirements must be placed on the uniformity of the film surface. Defects in the storage film affect not only the readout capability of the storage film, but also the capability of engaging the storage cells via x-ray radiation. They reduce the achievable image quality in both events. The achievable image quality therefore significantly depends on the freedom from defects.
Storage films are exposed to various mechanical stresses in x-ray diagnostic applications. For example, they are used in film cassettes in order to generate diagnostic x-ray exposures in medicine. Film cassettes are used in “over-table” apparatuses in which the patient to be examined is irradiation by x-ray radiation from above, with the patient lying on the cassette and exerting a two-dimensional pressure on the cassette and therewith on the storage film. The storage film is mechanically stressed.
Moreover, contact with the patient leads to the creation of moisture on the surface of the storage film, requiring the surface to be cleaned from time to time with a fluid-saturated cloth in order to remove adhering contaminants which likewise lead to the attachment of moisture. The quality of the storage film also suffers under the increase of the humidity.
“Needle image plates” (NIP), in which the luminophore is grown on a substrate in needle-shaped structures, are primarily used as storing luminophore layers. The needle tips of these structures end in the surface of the storage film and influence the x-ray sensitivity and storage capability of the film. Given support of the patient or subject to be examined on a needle image plate, the needle ends situated in the surface are mechanically stressed and can thereby be deformed. The x-ray sensitivity and the storage capability suffer under the deformation. Needle image plates therefore require a particularly effective mechanical surface protection.
From German patent document DE 100 48 810 A1, it is known to protect the surface of needle image plates in that a deformable damping layer is applied on the film surface. The damping layer thereby effects a uniform distribution of mechanical loads and must, for its part, be protected from scratches in order to not lose optical quality. For this purpose, it is proposed to apply a further cover layer made from SiO2, Al2O3, TiO2 or made from silicate. While the damping layer itself exhibits good bonding properties with the needle image plate, upon application of the further cover layer, bonding problems occur with the cover layer that can only be remedied via extremely elaborate production methods—if at all. Should a parylene layer (poly-para-xylylene) be used as a damping layer due to its excellent properties, this fails to achieve a sufficient bonding of the cover layer.